This invention relates generally to spiral baluns, and more particularly to a monolithically integrable and compact spiral balun that occupies less substrate area without a degradation in performance.
Circuits such as balanced mixers have traditionally utilized hybrid couplers, transformers, or baluns to achieve equal amplitude division and phase differential at the inputs of non-linear devices such as diodes, field-effect-transistors, etc. For example, a balun is utilized to provide balanced outputs from and unbalanced input, and for wireless applications, this requires (1) half the input signal amplitude at each of two outputs 180 degrees out of phase with each other and (2) very high frequency operation and low power consumption. While in principle, transformers can provide the required output signals, they are generally wire-wound devices that are frequency limited. Active baluns meet the frequency requirements but are generally characterized by high power consumption. Passive baluns, however, meet both requirements; i.e. high frequency at low power. A known Marchand balun comprises three transmission lines that are formed of a conductive material, preferably a deposited metal. An unbalanced input signal is applied to an input terminal at one end of a first transmission line, the other end of which is open; i.e. an open circuit or high impedance output. First ends of the second and third transmission lines are typically grounded, and second ends of the second and third transmission lines are coupled respectively to provide output signals that are substantially equal in amplitude and approximately 180 degrees out of phase with each other at a desired frequency. The balun is realized by two coupled line sections having an electrical length that is preferably approximately 90 degrees at the frequency of interest. In this case, xe2x80x9ccoupledxe2x80x9d refers broadly to placing two unshielded transmission lines in close proximity such that RF power can be transmitted between the lines due to the interaction of the electromagnetic fields of each transmission line. Coupled transmission lines often consist of three conductors (i.e. two transmission lines, plus a grounded plane) in close proximity, although more conductors could be used. Unfortunately, this design consumes a significant amount of die area when fabricated in monolithic form. This is unacceptable in the case of, for example, a mobile communication application where portability is stressed and downsizing of components and integrated circuits is required.
In view of the foregoing, it should be appreciated that it would be desirable to provide a compact, spiral balun for use in, for example, a double balanced, spiral balun mixer, which occupies less die area and therefore reduces cost without sacrificing performance. Additional desirable features will become apparent to one skilled in the art from the foregoing background of the invention and the following detailed description of a preferred exemplary embodiment and the appended claims.
In accordance with the teachings of the present invention, there is provided a spiral balun which includes first, second and third transmission lines on a substrate. The first transmission line has a first end coupled to receive an input signal and has a second end, the first transmission line having a first section proximate the first end forming a spiral in a first direction and a second section proximate the second end forming a spiral in a second direction. The second transmission line has a first end for providing a first output and a second end for coupling to a first potential. The second transmission line forms a spiral that interleaves the first section of the first transmission line and forms a spiral that winds in the second direction from its first end to its second end. Third transmission line has a first end for providing a second output and a second end coupled to a second potential. The third transmission line forms a spiral that interleaves the second section of the first transmission line and winds in the first direction from its first end to its second end.